[profile] Remove useless msync when dumping gcda files

Summary:
According the mmap man page (https://man7.org/linux/man-pages/man2/mmap.2.html) is only required to precisely control updates, so we can safely remove it.
Since gcda files are dumped just before to call exec** functions, dump need to be fast.
On my computer, Firefox built with --coverage needs ~1min40 to display something and in removing msync it needs ~8s.

Reviewers: void

Subscribers: #sanitizers, marco-c, sylvestre.ledru

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D81060

compiler-rt/lib/profile/GCDAProfiling.c

@@ -303,16 +303,11 @@ static void unmap_file() {
 
   mmap_handle = NULL;
 #else
-  if (msync(write_buffer, file_size, MS_SYNC) == -1) {
+  if (munmap(write_buffer, file_size) == -1) {
     int errnum = errno;
-    fprintf(stderr, "profiling: %s: cannot msync: %s\n", filename,
+    fprintf(stderr, "profiling: %s: cannot munmap: %s\n", filename,
             strerror(errnum));
   }
-
-  /* We explicitly ignore errors from unmapping because at this point the data
-   * is written and we don't care.
-   */
-  (void)munmap(write_buffer, file_size);
 #endif
 
   write_buffer = NULL;